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Friday, November 25, 2011

Oracle ASM disk failure - Part 2

Introduction

In Part 1, I wrote about a scenario when ASM detects READ_ERRS/WRITE_ERRS and updates these columns in v$asm_disk for the ASM disk. The DBA has to explicitly drop the disk in ASM. This article is about a different scenario when ASM instance itself performs the 'drop disk' operation.

This post assumes that you are using ASM redundancy (Normal or High) and that you are not using ASMLib program. The commands and syntax could be different if you are using ASMLib.

Scenario

In this scenario, ASM drops the disk automatically. Furthermore, the READ_ERRS/WRITE_ERRS in v$asm_disk could be showing a value of NULL (instead of an actual count of READ or WRITE errors noticed).

The disk is already dropped by ASM instance. There is no need of an "alter diskgroup ...drop disk" command again. Instead the DBA has to work with the system administrator and physically locate the failed disk in the disk enclosure and remove it.Add the replacement disk

1)Get the replacement/new device name, partition it and change ownership to the database owner. For example let the disk path after partitioning be/dev/sdk12)select distinct header_status from v$asm_disk where name = '/dev/sdk1';(Must show as CANDIDATE)

3)alter diskgroup #name# add disk '/dev/sdk1';

4)ASM starts the re-balancing operation due to the above disk add command. One can monitor the progress of the re-balance operation by checking v$asm_operation.

5)The disk add operation is now considered complete.How to decrease the ASM re-balance operation time

While the above ASM re-balancing operation is in progress, the DBA can let it complete quickly by changing 'ASM power' by running the below command for example.

alter diskgroup #name# rebalance power 8;The default power is 1 (i.e ASM starts one re-balance background process to handle the re-balancing work, called ARB process). The above command dynamically starts 8 ARB processes (ARB0 to ARB7), which can dramatically decrease the time to re-balance. The maximum power limit in 11g R1 is 11 (upto 11 ARB processes can be started).

Conclusion

I am not exactly sure why ASM shows the status of a failed disk in different ways, but these are two scenarios that I aware of so far.

None of the above maintenance operations (faile disk removal from the disk enclosure, new disk add) causes a downtime to the end user and therefore can be completed during normal business hours. The re-balance operation can cause slight degradation of performance and hence increase the power limit to let it complete quickly.

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